powerpc/pseries: Do not search for dma-window property on dlpar remove

The iommu_table pointer in the pci auxiliary struct of device_node has
not been used by the iommu ops since the dma refactor of
12d04eef, however this code still uses
it to find tables for dlpar. By only setting the PCI_DN iommu_table
pointer on nodes with dma window properties, we will be able to quickly
find the node for later checks, and can remove the table without looking
for the the dma window property on dlpar remove.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>